Such apparatuses are found, for example, in measurement devices and field devices in process measurements technology. These measurement devices are frequently employed in automation, and process control, technology for the ascertainment of a process variable, e.g. boundary surface, fill level, dielectric constant or some other type of physical and/or chemical process variable in a process flow. The assignee, for example, produces and sells measuring devices under the marks Levelflex and Multicap, which are primarily intended for determining and/or monitoring the fill-level of a medium in a container. In one of the large number of travel-time measuring methods, for example according to the guided microwave method, time domain reflectometry, or the TDR-measuring method (Time Domain Reflection), a high frequency pulse is sent along a Sommerfeld or Goubau waveguide or a coaxial waveguide and partially reflected back in the event of a DK-value (dielectric constant) discontinuity in the medium surrounding the waveguide. The fill-level can be ascertained from the time difference between the sending of the high-frequency pulse and the receipt of the reflected echo signal of the medium. The FMCW-method (Frequency Modulated Continuous Waves), in which the frequency range of a continuous measurement signal changes and the distance is measured via the frequency difference between the sent and the reflected measurement signals, is likewise performable in connection with the above measuring principle.
An additional measuring principle of the many different methods for ascertaining the fill-level in a container is the investigation of the capacitance change of a capacitive measurement setup with a medium-contacting measuring probe and the corresponding container wall or reference probe, if the degree of covering of the measuring probe with the medium, i.e. the fill-level of the medium in the container, changes.
Both of the techniques described above are so-called medium-contacting fill-level-measuring techniques, in which a measuring probe comes in direct contact with the medium to be measured. The measuring probe is normally secured in a container via a process connection, opening or nozzle, so that the measuring electronics are located outside of the process (i.e. not in contact with the medium), and the measuring probe is integrated in the process. Examples of these two procedures are described in more detail in the following references.
In European Patent, EP 1 069 649 A1, a simple construction of the waveguide for a fill level measuring device is shown, which unites the advantage of the one-wire and known multi-wire waveguides in that it exhibits no interaction with objects installed in the container, and can easily be cleaned of accumulated deposits. This was achieved in that the multi-wire waveguide is at least partially surrounded by a dielectric medium during the process, and thus no accretion can form between the individual wave guides.
In German Patent, DE 10157762 A1, a procedure is described for the capacitive fill-level measurement of a fill substance in a container with a measuring electrode and an evaluating circuit, in which the capacitive reactance is ascertained by means of two alternating voltage signals with different frequencies, thus eliminating the ohmic resistance.
In all known physical measuring principles, a compromise must, most often, be made with regard to the accuracy of the measurement and the reliability of the measurement. Therefore, on the basis of the environmental conditions and the properties of the medium, the most expedient physical measurement principle is selected, whose advantages outweigh its disadvantages. Fundamentally, there are employed in process-measurements technology as freely radiating measuring methods various physical measurement principles, such as microwave-travel-time measurement, ultrasound-travel-time measurement and gamma-ray-absorption measurement, as well as occasionally laser-light-travel-time measurement. As concerns medium-contacting procedures, the plumb-measuring method, the capacitive measuring method, the conductive measuring method and the guided-microwave measuring method, among others, are known. All of these measurement techniques have advantages over other measuring techniques according to the measuring principle, the measured medium, the measuring situation, the conditions of the process and the measurement performance.
A direct comparison of the various physical measuring principles shows that the selection of the measuring principle best-suited for the current application is in most cases very difficult. However, in the case of interface measurement, a large number of applications are covered by the highly developed medium-contacting measuring devices or combinations of these devices.
A fill-level measuring device in which both medium-contacting measuring techniques are employed is described the patent application DE 100 37 715 A1. In this publication, a fill-level measuring device is described whose medium-contacting measuring probe is connected with the capacitive measurement circuit or the travel-time measuring circuit via a change-over switch. The disadvantages of this design are that this change-over switch includes a semiconductor component which is located relatively near the measuring probe or the sensor in the signal path, and thus must be elaborately protected against electrostatic discharges. Moreover, an additional control line and a control signal for the switching of the change-over switch are required.